The invention relates to an arrangement and method for controlling a motor vehicle drive train in motor vehicles having an automatic transmission.
Such an arrangement is known in connection with a creep reduction, for example, from German Patent Document DE 10 2009 030 605 A1. According to this prior art, a function for the creep reduction is prohibited when the transmission control device recognizes the presence of a defined condition which makes it possible to infer that the driver has the intention to start driving dynamically.
A basic method for creep reduction is known, for example, from German Patent Document DE 44 46077 C2. This method relates particularly to automatic transmissions which are connected with the internal-combustion engine of the vehicle by way of a torque converter. The torque converter has an engine-side pump and a transmission-side turbine. The pump is coupled with the internal-combustion engine such that its rotational speed corresponds to the rotational speed of the internal-combustion engine. When the motor vehicle is stationary, the pump therefore drives the transmission-side turbine at a rotational speed corresponding to the idling rotational speed of the internal-combustion engine such that, without any creep reduction function, a so-called creep torque, which, among other things, is dependent on the rotational speed ratio of the rotational turbine speed to the rotational pump speed, on the transmission oil temperature, on the converter geometry and on the overall ratio, would occur at the output of the internal-combustion engine. With a creep reduction function, basically a creep (even though reduced) may also occur depending on the tractive resistance or rolling resistance and the extent of the creep reduction.
According to the creep reduction function known from German Patent Document DE 44 46077 C2, when the brake system of the vehicle is actuated, a value is detected which is proportional to the brake pressure. The creep reduction is controlled as a function of this detected value. In the case of the method known from DE 44 46077 C2, it is not only determined for controlling the creep reduction whether, but also how, extensively the vehicle brake system is actuated. An oscillating switching-on and off of the creep reduction can thereby be prevented when the vehicle brake system is actuated intermittently only at a low pressure, from which it can be inferred that specifically a creeping of the motor vehicle is desired, for example, in a traffic jam. However, when a considerable brake pressure is detected, preferably within a specific time window, the driver's intention of actually stopping the motor vehicle will be recognized and this wish is met by switching on the creep reduction.
The creep reduction or creep prevention is implemented, for example, by upshifting operations starting from a momentarily preselected gear, or by reducing the pressure in the clutch (starting gear) responsible for the momemtarily preselected gear.
The known prior art mainly concerns the emission reduction when the vehicle is stationary.
It is an object of the invention to improve a control of a motor vehicle drive train, particularly at a low driving speed, with a view to increasing comfort and the reproducibility for different transmission oil temperatures and for different idling rotational speeds of the internal-combustion engine.
This and other objects are achieved according to the invention by an arrangement and method for controlling a motor vehicle drive train consisting of at least one driving motor, a torque converter and an automatic transmission. The driving motor and the automatic transmission, including the torque converter, are controlled or automatically controlled by at least one electronic drive control device. For example, a first drive control device in the form of an electronic motor control device may be provided for the control of the driving motor and a second drive control device in the form of an electronic transmission control device may be provided for the control of the automatic transmission. However, a central drive control device may also be provided in which all control functions concerning the control of drive torques are combined, for example, in the form of a so-called torque structure. The invention can be used in vehicles having a single driving motor (such as an internal-combustion engine) as well as in hybrid vehicles having at least two driving motors (for example, an internal-combustion engine and an electric motor). According to the invention, a function module for implementing creep control is implemented in at least one drive control device.
This creep control is activated independently of a brake actuation when the actual rotational speed of the driving motor is greater than a defined desired rotational speed of the driving motor and when the actual rotational speed of the driving motor cannot be reduced as a result of at least one defined condition as a minimally possible rotational speed of the driving motor.
The invention is based on the following considerations. As a rule, automatic transmissions are equipped with a hydraulic torque converter as a start-up element. Among other things, the hydraulic power transmission of the converter is quadratically dependent on the differential motor-turbine rotational speed. As a function of the hydraulic power transmission, as well as of the converter excess, of the torque converter, the rotational motor speed and the overall ratio of the drive train, a defined creep torque is obtained, for example, for the creeping or maneuvering operation in drive position D or R.
In the case of engines with a higher idling rotational speed, such as 3-cylinder engines, a clearly increased creep torque occurs because of the quadratic dependence of the hydraulic power transmission on the differential rotational speed of the engine—turbine. This is felt by the driver by means of an unusually strong impulse of the vehicle to move forward or backward, which the driver can counter only by an increased brake pressure. Specifically, with respect to a maneuvering situation, this results in a clearly poorer proportioning capacity and in reduced comfort. Furthermore, in all situations involving increased idling rotational speed, such as a cold start, an activated air conditioner, an increased desire to charge the electric onboard power supply, or the like, there will again be an increased creep torque. It is a further disadvantage that vehicles with an automatic transmission and a torque converter cannot roll back, for example, on slight inclines, because of the constantly applied creep torque, without a changing of the drive position by the driver, thus, for example, from D to R. Vehicles with a start-up clutch, as a rule, are capable of rolling forward and backward on slopes solely by actuating the accelerator pedal.
A method is therefore provided according to the invention in order to automatically control the creep torque in the case of an automatic transmission having a torque converter. For this purpose, a frictionally engaged shifting element (at least a clutch or brake) in the transmission is to be operated or automatically controlled in a slip mode. The shifting element operated in the slip thereby determines the torque reaching the output and therefore the vehicle acceleration. The rotational slip speed in the shifting element is obtained from the resulting rotational speed ratio from the hydraulic power transmission at the torque converter.
Ideally, that shifting element can be used as the shifting element that is used for the so-called neutral idle control (NIC) function. The NIC function is a disengaging of the shifting element during a standstill or when coasting in order to increase the transmission efficiency by reducing the converter rotational slip speed and thereby the power loss. As a rule, the NIC function is exited by way of the reduction of the brake pressure by the driver.
As a result of the new function, the NIC shifting element exiting the NIC would directly change into the automatically controlled operation. For this purpose, the triggering or automatic controlling of the shifting element or the automatic adhesion control of the at least one clutch can take place as a function of various input parameters, such as the accelerator pedal, the engine torque, the rotational speed of the driving motor, the tractive resistance, the rotational output speed, the vehicle velocity, etc.
In an advantageous further development, a complete software module (function module) is provided, which coordinates all functions that are demanded from the shifting element to be disengaged, such as NIC; start-up support, MSA (=automatic motor start-stop system) and now, additionally, the automatic creep torque control or creep control.
The advantages can be combined as follows:                (a) Implementability of a low creep torque in the case of engines with a high idling rotational speed and automatic transmission with a hydraulic torque converter;        (b) Implementability of an arbitrary creep torque in the case of hybrid vehicles with one electrically driven axle and one axle driven by way of an automatic transmission by an internal-combustion engine;        (c) Implementability of an non-variable creep torque independently of temporary conditions with a higher idling rotational speed (for example, the air conditioner, warming-up, charging condition, particle filter regeneration, recuperation when stationary in the case of a hybrid, . . . )        (d) Permitting the “creep on demand” function as currently implemented in Applicant's series M3 vehicles with DKG;        (e) Improved proportioning capacity in the maneuvering, parking, and stop-and-go operation;        (f) Creaking noises when stopping the vehicle by way of the brake can be minimized; and        (g) Implementability of a rolling-back function on slopes as a function of the accelerator pedal position.        
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.